1. Field of the Invention
The invention relates to frequency synthesized oscillators, and more particularly relates to injection locked, synthesized oscillators.
2. Description of the Prior Art
Frequency synthesizers or synthesized oscillators are combinations of circuits and devices which synthesize signals for output at various frequencies. Synthesizers are common to both receiving and transmitting circuits, where they serve as carrier oscillators and local oscillators for mixers. Synthesizer frequencies may be fixed or tunable, either continuously or in steps. Spectral purity and frequency stability are critical design criteria for any frequency synthesizer or synthesized oscillator.
There are three basic types of frequency synthesizers in use today. These include the direct analog synthesizer, the indirect analog synthesizer which utilize phase locked loops, and the direct digital synthesizer.
Direct analog synthesizers utilize multiple RF techniques to translate and multiply reference frequencies to a desired frequency range. Frequency resolution using such methods, however, is generally poor, requiring many combinations of mixers and dividers to make small improvements in resolution.
Indirect synthesizers (phase locked loops) use analog voltage controlled oscillators as the primary frequency generating device. By comparing the output phase/frequency of the voltage controlled oscillator with a stable reference, i.e., a crystal oscillator, an error signal is produced to precisely control the output frequency. The spectral purity of output signals generated by a phase locked loop is generally very high, because the loop acts as a narrow tracking filter suppressing large amounts of noise inside the loop bandwidth. The narrow loop bandwidth, however, limits synthesizer tuning speed, which is a major limitation.
Direct digital synthesizer technology offers the simplest architecture of the three approaches, utilizing high speed digital circuitry to numerically generate a frequency stable sinewave pattern. Direct digital synthesizers may be used to quickly generate sinusoidal signals to a fraction of a hertz, the generated signals displaying low phase noise. Direct digital synthesizers may also be used to implement varied linear sweep and chirp signal generation. The sweep rate and linearity of the output is primarily a function of the quality of the system clock source.
FIG. 1 shows a direct digital synthesizer 1 comprising three basic elements: a phase accumulator 2, a sine look up ROM 4 and a digital-to-analog (D/A) converter 6. Direct digital synthesizer 1 digitally integrates incremental phase changes at a higher clock frequency than the frequency of the desired sinusoidal signal within phase accumulator 2. The synthesizer 1 then converts the resulting phase information to sinusoidal amplitude with sine look up ROM 4 and converts the digitized sinewave to an analog voltage with a D/A converter 6 (see FIG. 1A).
Current direct digital synthesizer technology is limited by upper clock frequency bounds of about 1 GHz. As a result, maximum direct digital synthesizer output frequency signals presently span approximately 240 MHz. To operate at higher frequencies, mixer conversions and multiplication may be used to increase the output bandwidth. Multiplication, however, affects discrete spurious tones, i.e., spurious signals. For example, multiplication by a factor of N increases the power level of spurious signal components by N.sup.2, lowering the spectral purity of the output signal.
Spurious frequency components or spurs are often found "close-in" to a carrier signal. Even a perfect D/A converter used within a direct digital synthesizer will produce spurious frequency components. This is because each discrete amplitude sample of the fabricated waveform is a quantized approximation of the ideal value. Consequently, digital-to-analog converter errors are unavoidable, showing themselves as harmonics, intermodulation products, and spurious signals, very close in frequency to the desired signal.